1. Field of the Invention
This invention relates in general to liquid crystal display devices and more particularly, to the display devices comprising cholesteric liquid crystals having bistable characteristics and provided with birefringence medium layers.
2. Description of the Related Art
Liquid crystal display devices typically comprise a layer of liquid crystals placed between a pair of light transparent substrates provided with alignment films and transparent electrodes, a pair of polarizers disposed on outward surfaces of the substrates, whereby constituting liquid crystal cells.
Several liquid crystal display devices using bistable cholesteric liquid crystals have been developed (for example, Japanese Patent H1-51818). Liquid crystal cells of the display device include a layer of cholesteric liquid crystals which are constituted to have an intrinsic helical pitch of approximately twice as large as a thickness of the liquid crystal layer.
In addition, the liquid crystal cells have a bistable character and can be switched between two states by an application of voltage, such that a twist angle of the liquid crystal layer along the layer thickness is either approximately 360xc2x0 or 0xc2x0, corresponding to a twisted state or a uniform state (non-twisted state), respectively. It may be noted that the latter value (approximately 0xc2x0) is not necessarily 0xc2x0 but a value subtracted 360xc2x0 from the above-mentioned xe2x80x9capproximately 360xc2x0 xe2x80x9d.
The pair of polarizing plates are each provided on an upper and a lower faces of the cell, whereby constituting the liquid display device, as aforementioned.
When the polarizing plates are each arranged such that a transparency axis of one plate makes a right angle to the other, and that a direction of the liquid crystal alignment at the uniform state makes a 45xc2x0 angle to the transparent axis of polarizing plate, birefringent colors are generally observed.
These colors are not preferable for black and white displays, and a white display color is obtained by adjusting a xcex94nd value of the device to about 270 nm, where xcex94n and d represent an optical anisotropy of the liquid crystal and a thickness of the liquid crystal layer, respectively. Although some birefringence may also arise in the twisted state, this results in a nearly black display color due to relatively small values of the birefringence in that state, without significantly affecting display qualities.
As above-mentioned, display colors of nearly black and white quality can be obtained by conventional bistable liquid crystal display devices using cholesteric liquid crystals. However, due to the birefringence in the twisted state of the liquid crystal, in practice, there exist a certain amount of the light which breaks through display devices in black display state, resulting in an insufficient contrast. In addition, when these liquid crystal cells are used as color displays equipped with color filters, the above-mentioned coloration gives rise to serious problems, and satisfactory color purity has not been achieved.
It is therefore an object of the present invention to provide liquid crystal display which overcomes the above-noted difficulties.
A further object of the present invention is to provide a display device with a high contrast and satisfactory color quality when used as color displays equipped with color filters, and also to achieve display characteristics such as a higher duty ratio operation and faster time response, suitable for displaying a large volume of information data.
These and other objects of the present invention are accomplished by the provision of a liquid crystal display device including a cholesteric liquid crystal layer which has an intrinsic helical pitch of approximately twice as large as a thickness of the liquid crystal layer, which is disposed between a pair of substrates and has a bistable character capable of switching by an application of voltage. The display device also includes a pair of polarizers each provided on a upper and lower faces of a liquid crystal cell.
In the present invention, at least one birefringent medium layer is further provided between a polarizer and a substrate for either one of a upper or lower set of the polarizer and the substrate. This results in a reduction of birefringent effect of the display device at either one of the bistable states, whereby solving the afore-mentioned problems of the conventional display devices.
According to another aspect of the present invention, the birefringent medium layer has a twist angle which has approximately the same magnitude as, and an opposite direction to, that of the liquid crystal layer, a xcex94nd value approximately equal to that of the liquid crystal layer, and an approximately right angle between a slow axis of the birefringent medium layer and an alignment direction of a face opposing to the birefringent medium layer, of the liquid crystal cell.
According to yet another aspect of the invention, the birefringent medium layer is nearly uniaxial, has a slow axis in the plane of the birefringent medium layer and a xcex94nd value approximately equal to that of a liquid crystal cell. In addition, the birefringent medium layer is provided so as to have an approximately right angle between a slow axis of the birefringent medium layer and an alignment direction of the face of the liquid crystal, opposing to the birefringent medium layer.
According to another aspect of the invention, the birefringent medium layer is selected from the group consisting of a stretched or extruded polymer film, a liquid crystal cell having a homogeneous alignment, an aligned polymer liquid crystal, or an alignment-immobilized liquid crystalline polymer which is obtained by freezing molecules of the polymer liquid crystal aligned homogeneously.
By this construction of the liquid crystal display, the birefringent effect can be reduced, whereby achieving a high contrast, satisfactory color purity, and other display characteristics such as a higher duty ratio operation and faster time response, suitable for displaying a large volume of information data.